The atomic layer deposition (ALD) has been proposed as an alternative to the CVD-based deposition method for the formation of a silicon nitride film or a silicon oxide film, such as an Si3N4 film or an SiO2 film. The ALD is a surface controlled process conducted in a surface kinetic manner, which results in the two-dimensional layer-by-layer deposition on the wafer surface. The precursors of silicon nitride, such as the DCS (dichlorosilane, SiH2Cl2) gas and ammonia (NH3) gas, flow into a tube for forming the silicon nitride film.
The deposition of the silicon nitride film according to the ALD process is preformed by repeating a cycle until a desired film thickness is obtained. The cycle includes the following steps. Firstly, a deposition gas, which contains the DCS gas as a silicon source, is supplied to a processing vessel such that the silicon source can be adsorbed by the wafer surface. Secondly, the nitrogen gas is supplied to drive out the DCS gas. Thirdly, a nitride gas, which contains the ammonia gas radicalized by the plasma, is supplied to the processing vessel such that the adsorbed DCS can be decomposed and nitrified. Fourthly, the nitrogen gas is supplied to drive out the ammonia gas. However, the above-mentioned steps will result in a delivery problem due to the very low vapor pressure of the precursors as well as a pattern loading effect. There is a need to solve the above problems.